The present invention relates to an exposure apparatus and exposure method, and in particular those used in photolithography for manufacturing devices such as semiconductor devices, image pickup devices, liquid crystal display devices and thin film magnetic heads.
When manufacturing semiconductor devices and the like, either a static exposure-type (e.g., stepper) or a scanning exposure-type (e.g., step-and-scan system) projection exposure apparatus is used to transfer the image of the pattern of a reticle as a mask onto a wafer (or glass plate and the like) coated with resist, via a projection optical system. With the advance of finer patterns in semiconductor integrated circuits and the like, it is desirable to improve the resolving power of the projection optical system provided in such exposure apparatus. This can be accomplished by shortening the exposure wavelength or increasing the numerical aperture (N.A.)
The g-line (436 nm wavelength) to the i-line (365 nm wavelength) of a mercury lamp have principally been used in recent years for the exposure light (exposure energy beam). Recently, however, exposure light of shorter wavelength, e.g., KrF excimer laser light (248 nm wavelength), as well as light in the deep ultraviolet region and the vacuum ultraviolet region of an ArF excimer laser (193 nm wavelength) or F2 laser (157 nm wavelength) are being employed.
Projection optical systems using an exposure energy beam in the ultraviolet region below 200 nm are proposed in, for example, Japanese Patent Application Kokai No. Hei 5-173065, and U.S. Pat. Nos. 5,402,267 and 5,668,672.
The optical systems proposed in the above references include refractive optical elements made of synthetic silica (SiO2). If an exposure energy beam in the ultraviolet region under 200 nm is used as the exposure light, there is a risk the synthetic silica, which includes oxygen (O2), will absorb the exposure energy beam in this wavelength region. This is because this wavelength region is near the absorption band of oxygen. In addition, there is also a risk that contamination in the manufacturing process of the synthetic silica by impurities will reduce the transmittance (i.e., increase the absorptance) in this wavelength region.
Absorption of the exposure energy beam in this wavelength region by a synthetic silica optical members will produce heat. This, in turn, can lead to a change in the shape of the surface of the optical members due to thermal expansion, or a change in the refractive index of the silica itself. If this type of fluctuation is produced by the exposure energy beam, the performance of the projection optical system will deteriorate, making it difficult to transfer a fine pattern.
On the other hand, advances have been made in narrowing the spectral bandwidth of the light source that supplies the exposure energy beam in the wavelength region under 200 nm. However, in actuality, the exposure energy beam has a finite bandwidth. Accordingly, the correction of chromatic aberration in a projection optical system is still essential for transferring the pattern on a mask onto a substrate while maintaining adequate contrast.
The optical members in the projection optical systems of the abovementioned Japanese Patent Application Kokai No. Hei 5-173065 and U.S. Pat. No. 5,402,267 are made of only one type of silica. Thus, there is a risk of deterioration in imaging performance due to fluctuations in irradiation if used in combination with a light source that supplies an exposure energy beam with a wavelength under 200 nm. In addition, chromatic aberration in U.S. Pat. No. 5,668,672 is corrected by combining silica and fluorite lenses. However, since silica exists in the projection optical system, there is a risk that imaging performance will deteriorate due to fluctuations in irradiation if used in combination with a light source that supplies an exposure energy beam with a wavelength under 200 nm. Thus, the transfer of fine patterns is problematic in the systems disclosed in the above references.
The present invention relates to an exposure apparatus and exposure method, and in particular those used in photolithography for manufacturing devices such as semiconductor devices, image pickup devices, liquid crystal display devices and thin film magnetic heads.
Accordingly, a first goal of the present invention is to reduce the absorption of the exposure energy beam by the optical members in the projection optical system to a level at which there is substantially no effect, and to transfer extremely fine patterns without producing fluctuations in irradiation due to changes in the optical properties of the optical members induced by the exposure energy beam.
A second goal of the present invention is a method of manufacturing the exposure apparatus according to the present invention in a manner that provides an exposure apparatus that can transfer extremely fine patterns, resulting in devices having high-density patterns.
A first aspect of the invention is an exposure apparatus capable of transferring onto a wafer the image of a pattern on a reticle. The apparatus comprises a light source capable of supplying an exposure energy beam with a wavelength under 200 nm and an illumination optical system arranged to receive the exposure energy beam from the light source. The illumination optical system is designed to guide the exposure energy beam to the reticle. The apparatus also includes a projection optical system arranged between the reticle and the substrate, capable of forming an image of the reticle pattern onto the substrate based on the exposure energy beam passing through the reticle. The projection optical system has a plurality of refractive optical members, wherein at least two dioptric optical members of the plurality of refractive optical members are arranged along an optical path of the exposure energy beam, and wherein each refractive optical member in the plurality of dioptric optical members is made of at least two types of fluoride crystalline materials.
A second aspect of the invention is a method of exposing onto a substrate the image of a pattern provided on a reticle. The method comprises the steps of first, supplying an exposure energy beam with a wavelength under 200 nm, then guiding the exposure energy beam to the reticle and through at least two refractive optical members, then forming the image of the reticle pattern onto the substrate, wherein all refractive optical members positioned between the reticle and the substrate are made of at least two types of fluoride crystalline materials.
A third aspect of the invention is a method of manufacturing an exposure apparatus, including the steps of providing a light source capable of supplying an exposure energy beam having an optical path and a wavelength under 200 nm, then forming a first refractive optical element from a first fluoride crystal, then forming a second dioptric optical element from a second fluoride crystal different from the first fluoride crystal, and then arranging the first and second dioptric optical elements along the optical path of the exposure energy beam.